Disc drives are capable of storing large amounts of digital data in a relatively small area. A disc drive stores information on one or more spinning recording media. The recording media conventionally takes the form of a circular storage disk with a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across a surface of the disk by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft supported from a base plate, and a hub to which the spindle is attached having a sleeve into which the shaft is inserted. Permanent magnets, which are typically attached to the hub, interact with a stator winding to rotate the hub relative to the shaft. This description is consistent with a fixed shaft motor; however, the invention to be described below is as easily useable with a motor comprising a rotating shaft, an end of the shaft supporting the hub for rotation to support the rotation of the disks.
In either case, to facilitate rotation and for best drive performances, one or more bearings are disposed between the hub or sleeve and the shaft.
Over time, disk drive storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has led to greater emphasis on restrictive tolerances in the manufacturing and operation of magnetic storage disk drives. For example, to achieve increased storage density, read/write heads must be placed increasingly close to the surface of the storage disk.
As a result, the bearing assembly which supports the storage disk is of critical importance. A typical bearing assembly of the prior art comprises ball bearings supported between a pair of bearing races which allow a hub of a storage disk to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of insufficient damping.
An important measure of the operating effectiveness of a fluid dynamic bearing motor is the stiffness to power ratio, where stiffness gives the ability of the system to perform better dynamically, and power is a measure of power consumed to start rotation and maintain the constant speed and rotation of the motor. Most known fluid dynamic bearings today in commercial use are made with oil as the fluid which is maintained in the bearing gap between the two relatively rotating surfaces. This maintains the proper stiffness and damping of the bearing which reduce non-repeatable run-out due to shock and vibration; however, because of the relatively high viscosity of such fluids, especially at lower temperatures such as at startup, considerable power is consumed to establish and maintain high speed rotation.
Finally, to maintain the required axial and radial stiffness and damping of the bearing, some minimum length of a journal and width or diameter of a cone or surface area of a cone or width or diameter of a thrust plate must be devoted to grooved surface, against which pressure can come to bear to maintain the stiffness and damping of the system. Therefore, typically known bearing systems have had a plurality of fluid dynamic bearings in series. For example, known systems include two conical bearings spaced along a shaft in cooperating to provide both axial and radial stiffness and damping; or a shaft with a thrust plate, with the journal bearings on the shaft and the thrust bearings on the thrust plate being arrayed in series to operatively cooperate and maintain the stiffness and damping of the system. However, all of this leads to fairly high profile designs to accommodate these serially arrayed bearings; the smaller disk drives which are the designed target for use in portable computers and the like cannot accommodate high profile drives.